Subsea wells are frequently drilled using a floating drilling vessel such as a semi-submersible vessel using a subsea blowout preventer (BOP) stack mounted on the wellhead near the sea bed. Commonly, if a subsea tree is then installed, a subsea BOP is also used to run the tubing hanger.
Certain operators, in order to save cost, have come to drill subsea wells using a floater with a surface-type BOP located at the rig. A high pressure riser extends from the surface-type BOP stack to the subsea wellhead. This type equipment is satisfactory for drilling the well, but, can present a problem during completion of the well. In particular, when a tubing hanger is run with a tubing hanger running tool (THRT), the umbilical which provides control for the tool can be damaged or cut if the surface BOP is closed for any reason, e.g., to control the well in case of a kick or to close the rams to pressure test the tubing hanger after it is landed.
Because of safety concerns, a refinement to the surface stack drilling technique has been made in recent years wherein a simplified subsea stack is incorporated just above the wellhead. Normally, the set of rams in the subsea stack has only emergency control and is not routinely used for pressure control.